1. Field of the Invention
This invention relates to a speaker diaphragm and a method for producing the same. More particularly, it relates to a diaphragm exhibiting a superior modulus of elasticity due to extensive orientation of the ultra-high molecular weight polyolefin, and a method for producing the same.
2. Description of the Prior Art
In a speaker diaphragm, a demand for a high specific modulus of elasticity E/P, where E is a modulus of elasticity and .rho. the density is raised to enhance the range of piston movement ,while a demand is similarly raised for larger internal losses for smoothing frequency characteristics in the high sound range.
Until now, a method of mixing carbon fibers into the diaphragm material, mainly composed of paper pulp, for example, or a method of mixing a material of a higher modulus of elasticity, such as carbon fibers or mica, into the high molecular weight material, has been practiced as means for increasing the specific modulus of elasticity. On the other hand, the internal losses may be improved by applying an organic paint showing large internal losses, known as a dumping agent, to the surface of the diaphragm, or impregnating the diaphragm with such paint.
With the former method by mixing carbon fibers into paper pulp, it is difficult to achieve a high specific modulus of elasticity due to limitations imposed on the amount of the carbon fibers, whereas, with the method by mixing the material of the high specific modulus of elasticity into the high molecular weight material, the specific modulus of elasticity cannot be increased sufficiently because of increased melting viscosity at the time of molding and resulting limitations on molding. With the latter method, coating or impregnation of the dumping agent for improving the internal losses results in the lowered specific modulus of elasticity.
With this in view, ersearches and development of diaphragm materials having a high specific modulus of elasticity and large internal losses, such as ultra high molecualr weight polyolefin, are progressing.
Meanwhile, for realizing the high specific modulus of elasticity and large internal losses of the above mentioned ultra high molecular weight polyolefin, it is essential that the molecular chains of polyolefin be aligned in one direction. Thus the general practice has been to use fibers previously stretched by a gel stretching method as the diaphragm material.
For example, the Japanese Patent Publication No. JP.A58-182994(1983) dislcoses a technique of forming ultra high molecular weight polystyrene fibers with the propagation velocity of the longitudinal waves of not less than 4000 m/sec into a sheet by a wet paper making technique, drying and pressing the sheet by air under pressure to produce a speaker diaphragm.
However, with the diaphragm produced by the above mentioned paper making technique, since the fibers are oriented at random, the specific modulus of elasticity is markedly lowered as compared to the specific modulus of elasticity proper to the fibers.
On the other hand, the polyolefin fibers are difficult to mold on account of their inactivated surfaces, while the use of the binder has practically no effect in improving moldability of the polyolefin fibers.